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The session chooser was previously hardcoded to call /etc/X11/Xsession.
However, it turns out that file is Debian specific, so this change
introduces an Xsession script in the CRD directory and updates the
session chooser to call that. On Debian, this file just delegates to
/etc/X11/Xsession, but it makes it possible to install CRD on other
distros by swapping in an appropriate replacement.

Change-Id: I50c9599c929b1875798ba04043762358024607f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436624
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811945}